High-voltage outdoor bushings are known which include a conductor extended along an axis, a condenser core and an electrically insulating polymeric weather protection housing molded on the condenser core. The condenser core can contain an electrically insulating tape which is wound in spiral form around the conductor, capacitance grading insertions arranged between successive windings of the tape and a cured polymeric insulating matrix embedding the wound tape and the capacitive grading insertions. Such a bushing can be used in high voltage technology, such as in switchgear installations or in high-voltage machines, like generators or transformers, for voltages up to several hundred kV (for example, voltages between 24 and 800 kV).
A high-voltage outdoor bushing is a component that can be used to carry current at high potential from an encapsulated active part of a high-voltage component, like a transformer or a circuit breaker, through a grounded barrier, like a transformer tank or a circuit breaker housing, to a high-voltage outdoor line. In order to decrease and control the electric field the outdoor bushing includes a condenser core which facilitates the electrical stress control through floating capacitance grading insertions, which are incorporated in the condenser core. The condenser core decreases the electric field gradient and distributes the electric field homogeneously along the length of the bushing.
The condenser core of the bushing can be wound from kraft paper or creped kraft paper as a spacer. The capacitance grading insertions are executed as either metallic (e.g., aluminium) sheets or non-metallic (e.g., ink, graphite paste) patches. The insertions are located coaxially so as to achieve an optimal balance between external flashover and internal puncture strength. The paper spacer ensures a defined position of the insertions and the mechanical stability of the condenser core. The condenser core is impregnated with resin (RIP, resin impregnated paper). The resin is then introduced during a heating and vacuum process of the core. Such a RIP outdoor bushing can have an advantage that it is dry (oil free).
The outdoor bushing includes an outdoor side with an insulator made of either porcelain or a weather-resistant polymeric material, for example, on the basis of silicone or of epoxide, having sheds which ensure the creepage distance for withstand voltages under all operation conditions. The porcelain has been used as insulation material, however, there is a continuously growing desire for polymeric insulation. The desire for polymeric insulation is mainly based on the fact that polymeric insulators have the additional benefit of being hydrophobic (water repellent) which leads to a self cleaning property, and which thus extends service life and lowers significantly substation maintenance costs. Moreover, the silicone intrinsic hydrophobic property helps to break up water films and to create separate droplets which reduce leakage currents, prevent flashover and elevate the voltage withstand capability in wet and highly contaminated conditions, which exist in coastal or highly polluted environments. Furthermore, a bushing with polymeric insulation is lightweight and resistant against vandalism and earthquake. Besides such a bushing is explosion proof. Thus a scattering of a rigid insulating housing, for example, of a porcelain insulator, and a damage of secondary equipment is mostly excluded.
A high-voltage outdoor bushing with a conductor extended along an axis, a condenser core coaxially surrounding the conductor and with an electrically insulating polymeric weather protection housing is described in EP 1 284 483 A1. The weather protection housing is manufactured from a silicone and is directly molded on the outer surface and the high-voltage front face of the condenser core and is extended to a part of the surface of the conductor, which is not covered from the condenser core. A bushing cap which protects the high-voltage side against the weather becomes no longer necessary and thus the bushing can be manufactured with low costs. However, directly molded outdoor bushings have shown to generate significant problems during storage and operation. The dissipation factor tan δ has increased considerably during extended periods of storage and operation.
Further high-voltage outdoor bushings which respectively include a conductor extended along an axis and a condenser core coaxially surrounding the conductor are disclosed in EP 1 622 173 A1, EP 1 798 740 A1 and WO 2006/131011 A1. These bushings respectively include a composite insulator as weather protection housing which is designed as a prefabricated rigid housing. The rigid housing receives the prefabricated condenser core and the conductor and is closed by a cap and a mounting flange.
The production of the condenser core includes winding an insulating tape onto the conductor, adding capacitance grading insertions during winding between successive layers of the tape, placing the wound tape into a mold, applying a vacuum to a mold and impregnating the evacuated wound tape with an insulating material consisting of a polymer which is loaded with an inorganic filler powder. Afterwards the impregnated wound tape is cured. The resulting condenser core is cooled down and machined if desired. In order accelerate the impregnation, at least one of the layers of the tape (EP 1 622 173 A1) and/or one of the capacitance grading insertions (EP 1 798 740 A1) includes holes and/or the tape contains the inorganic filler particles which are pre-filled into the tape before execution of the impregnation process with the unfilled polymer (WO 2006/131011 A19).
Such high-voltage outdoor bushings can be expensive since the composite insulators are manufactured separately and include a bushing cap. Furthermore, electrically insulating material is used for filling gaps and pores within the bushing housings and for preventing electrical discharges and failures in the bushings.
High-voltage outdoor bushings with a condenser core of a moisture absorbing (e.g., hygroscopic) material are known from WO 2005/006355 A and GB 537 268 A. In these bushings the moisture uptake in the condenser core can be addressed by a diffusion barrier which is applied to the surface of the core and which includes a film having low water permeability, such as a solid moisture-proof skin.